Why not return plain text in both cases as a test instead of HTML? That way you can rule out any odd stuff in HTML and see if it's a perl/CGI issue.

My really old copy of "CGI Programming" has examples on pgs 103-4 for this sort of thing and is seems it should work. Looks like an issue with the server or the compiling into an exe and the server. As already mentioned what's in the logs.